Field of the Invention
The present invention relates to a matrix material for a carbon fiber-reinforced composite and a matrix material for an organic fiber-reinforced composite.
Description of the Related Art
A structure made of a fiber-reinforced composite (FRP) has a light weight and an excellent mechanical strength, and thus has been widely used in various fields. In general, the structure is produced via a FRP prepreg. A reinforcing fiber sheet, which includes a bundle (UD material) containing a plurality of reinforcing fibers arranged parallel to each other, a cross material containing reinforcing fibers woven horizontally and vertically, or the like, is impregnated with a matrix material to form a prepreg sheet. A plurality of such prepreg sheets are stacked and molded under heat and pressure, and then the matrix material is hardened to obtain the FRP structure.
It is preferred that the matrix material contains an epoxy resin as a resin component (matrix resin) from the viewpoints of strength, heat resistance, etc. The reinforcing fiber is preferably a carbon fiber or an organic fiber.
For example, a production method described in Japanese Laid-Open Patent Publication No. 09-208838 may be used in a case where a carbon fiber is impregnated with a matrix material containing a matrix resin of a thermosetting resin such as an epoxy resin to form the prepreg. First, the matrix material is thermally softened and applied to at least one of two release papers, whereby a resin film containing the matrix material is formed on the release paper. Then, the softened resin film is brought into contact with the carbon fiber and is pressed between the release papers. The carbon fiber can be impregnated with the matrix material to obtain the prepreg in this manner.
In order to produce a structure having an excellent property such as a high strength from the prepreg, a satisfactory amount of the matrix material has to be sufficiently introduced into the inside of the carbon fiber to achieve a good property in the prepreg. The viscosity of the matrix material in the softened state (which may be referred to simply as the viscosity of the matrix material) may be lowered to improve an impregnation property of the matrix material. However, when the viscosity of the matrix material is excessively lowered, the matrix material may flow out from the carbon fiber, so that the matrix material amount may be insufficient in the prepreg disadvantageously. In addition, the matrix material having the low viscosity may readily adhere to another member or the like, so that the handling property of the prepreg may be deteriorated disadvantageously.
When the viscosity of the matrix material is excessively increased to avoid the above problems, the impregnation property is deteriorated. Consequently, it is difficult to achieve improvement of the impregnation property of the matrix material (the matrix resin) while preventing the matrix material from flowing out, for the purpose of preparing the prepreg where the carbon fiber is sufficiently impregnated with the satisfactory amount of the matrix material.
In above-described Japanese Laid-Open Patent Publication No. 09-208838, a technology using a matrix material with a viscosity after the impregnation higher than a viscosity during the impregnation is proposed to achieve both of the improvement of the impregnation property of the matrix material and the prevention of the flow out. Specifically, in the production method described in Japanese Laid-Open Patent Publication No. 09-208838, the matrix material for the prepreg contains a matrix resin, a radical-polymerizable unsaturated compound, and a polymerization initiator capable of generating a radical under heating. Thus, first, the carbon fiber is impregnated with the matrix material having a low viscosity and a high impregnation property before the heat treatment. Then, the heat treatment is carried out to form a polymer from the radical-polymerizable unsaturated compound, whereby the viscosity of the matrix material is increased after the impregnation. As a result, the prepreg can be obtained with improved handling property while preventing the flow out of the matrix material.
In particular, the prepreg and the structure prepared using the UD material of the carbon fiber exhibit a lower strength in a direction perpendicular to the fiber direction than in the fiber direction. Furthermore, in a case where a plurality of the prepreg sheets prepared using the UD material or the cross material are stacked, the resultant structure exhibits a lower strength between the sheets in the stacking direction than in the fiber direction. Thus, the CFRP structure exhibits a relatively low strength in a particular direction, and has an anisotropic strength property. However, in the case of using the structure as a member of an aircraft or the like, the structure is required to have an extremely excellent strength property, etc. Therefore, it is necessary to sufficiently improve the strength of the structure in the particular direction mentioned above.
The strength property of the entire structure, including the strength in the particular direction, can be improved by increasing the interface adhesion between the carbon fiber and the matrix material to suppress the strength reduction due to the anisotropy. Japanese Laid-Open Patent Publication Nos. 2011-214209 and 2006-249395 each propose a method for increasing the adhesion.
The technology of Japanese Laid-Open Patent Publication No. 2011-214209 contains subjecting a carbon fiber bundle to a surface treatment. In this treatment, a sizing agent of a mixture containing a first compound and a second compound at a particular ratio is applied to the surface of the carbon fiber bundle. More specifically, in this treatment, the first and second compounds are dissolved in an organic solvent or water to prepare a solution of the sizing agent, the carbon fiber bundle is immersed in the solution, and the organic solvent and the like are evaporated and removed. The first compound has a hydroxyl group and/or a carboxyl group with a primary amino group and/or a secondary amino group, and the second compound has an epoxy group.
In Japanese Laid-Open Patent Publication No. 2006-249395, a carbon fiber is surface-treated with a sizing agent containing a vinyl ester resin or the like, and an unsaturated matrix resin and an epoxy resin are mixed to prepare a matrix material. In the surface treatment, the surface of the carbon fiber is subjected to an oxidation treatment and then impregnated with the sizing agent using an immersion roller, etc. The unsaturated matrix resin may be a vinyl ester resin or the like, and the epoxy resin may be a bisphenol A-type epoxy resin or the like.
On the other hand, in the case of using the organic fiber as the reinforcing fiber, the organic fiber is advantageous over inorganic fibers in that the cost of the structure can be easily reduced. Furthermore, for example, an aramid fiber or the like can be used as the organic fiber to produce the structure with excellent strength, impact resistance, etc.
Accordingly, as the matrix material for forming the prepreg using the organic fiber as the reinforcing fiber, for example, an epoxy resin composition containing an epoxy resin as the matrix resin is proposed in Japanese Laid-Open Patent Publication No. 2003-321557.
This epoxy resin composition contains components A to C of epoxy resins and a component D of a thermoplastic resin to improve the adhesion and flexibility of the prepreg. Specifically, the component A is an epoxy resin that has a viscosity of 300 poise or less at 25° C., has an epoxy equivalent weight of 300 or less, and is in the liquid state at room temperature. The component B is a phenol novolac epoxy resin. The component C is an epoxy resin that has an epoxy equivalent weight of 400 to 2000 and is in the solid state at room temperature. The component D is a thermoplastic resin such as a polyvinyl acetal resin soluble in each of the components A to C.